Energy Information Document 1028

Energy Information Document 1028

Chapter IX. Landscaping to Conserve Energy: A Guide to Microclimate Modification1 Alan W. Meerow and Robert J. Black2

INTRODUCTION The world’s petroleum reserves are not a limitless resource. Oil supplies continue to dwindle as the world’s population grows and more developing countries establish an industrial base. As progress continues in the development of alternative energy sources, energy consumption must be reduced through effective conservation practices. In northernmost Florida, temperatures fall below freezing on an average of 15 to 25 days and in Miami, winter fronts with associated winds cause uncomfortably cool temperatures for short periods of time but Florida’s subtropical climate reduces the demand for winter heating (Barrick and Black, 1984). However, Florida’s long, hot and humid summers create a high demand for air conditioning during 5 to 7 months of the year. As much as 30 percent of the cost of keeping a home comfortable could be saved by effective management of the microclimate that surrounds it. Microclimate is the term used to define any small, local area within which the effects of weather are both relatively uniform and easily modified. The "passive" methods of microclimate modification discussed in this chapter are simple, low-cost improvements that can

decrease the energy costs associated with maintaining interior comfort. Microclimate modification involves the best use of structural and landscape design elements to maximize or moderate sunlight, shade and air movement. Structural modifications involve the design of the house and associated construction (walkways, fences, patios). Landscape modifications involve the use of plants to further increase or decrease the impact of sun and wind upon the local environment.

STRUCTURAL ELEMENTS OF MICROCLIMATE MODIFICATION When choosing or designing a new home, several decisions can be made that will strongly influence the degree to which interior comfort will require heating or air conditioning. The homeowner should incorporate effective insulation in ceilings and walls, and weatherstrip around windows and doors, even if local ordinances do not require such practices. Total energy savings of 50-60 percent can be realized if conservation practices are followed in new home construction (Buffington, 1979). In Florida’s hot and humid climate, a house will be more energy efficient if it is oriented with the long axis

1.

This document is Chapter 9 of the Energy Information Handbook, Energy Information Document 1028, a series of the Florida Energy Extension Service, Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida. Revised: August 1991.

2.

Alan W. Meerow, Assistant Professor, Agricultural Research and Education Center, Ft. Lauderdale; Robert J. Black, Associate Professor, Department of Environmental Horticulture, Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida, Gainesville FL 32611. The Florida Energy Extension Service receives funding from the Florida Energy Office, Department of Community Affairs and is operated by the University of Florida’s Institute of Food and Agricultural Sciences through the Cooperative Extension Service. The information contained herein is the product of the Florida Energy Extension Service and does not necessarily reflect the views of the Florida Energy Office.

The Institute of Food and Agricultural Sciences is an equal opportunity/affirmative action employer authorized to provide research, educational information and other services only to individuals and institutions that function without regard to race, color, sex, age, handicap, or national origin. For information on obtaining other extension publications, contact your county Cooperative Extension Service office. Florida Cooperative Extension Service / Institute of Food and Agricultural Sciences / University of Florida / Christine Taylor Stephens, Dean

Chapter IX. Landscaping to Conserve Energy: A Guide to Microclimate Modification

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running east-west (Figure 1). With this orientation, only the short walls of the house receive the direct morning and afternoon sun, thereby reducing the total heat load on the structure. An east-west orientation can save as much as $75 to $100 per year in cooling costs alone for the average home. In the winter, when the sun is lower in the sky, the south-facing long wall receives the heating benefits of solar radiation. Divergence of up to 10 degrees in either direction from this orientation is allowable to compensate for the prevailing wind direction or other specific site requirements.

Figure 1. In Florida’s hot, humid climate, a house will be more energy efficient if it is oriented with the long axis running east-west.

Light colored materials reflect sunlight; dark materials absorb the radiation. A house with dark walls and roof is less expensive to heat in winter, but more costly to cool in summer. Light colored walls and roofs lower cooling costs but increase the need for winter heating. In Florida, the use of light colored materials is more cost effective and energy efficient, since the cooling season is considerably longer than the heating season. Fencing is primarily used around homes to ensure privacy or mark boundaries. Fencing also directly influences the patterns of air circulation and air movement can affect the energy efficiency of the home, depending on the season of the year, direction of prevailing winds, and degree of dependence on air conditioning for home cooling. Air movement around the home may raise energy consumption by increasing conductive heat loss (in winter) and heat gain (in summer) through walls and windows, and infiltration of outside air around the edges of windows and doors.

Figure Figure3.2.Open A solid fencing fence maximizes on thesummer north side air flow. of

the house provides a barrier against cold winter winds.

In Florida, winter’s prevailing winds are from the north or northwest. Thus, a solid fence on the north side of a house can provide a barrier against cold winter winds (Figure 2). Southerly winds predominate during the warmer months of the year when effective air circulation is generally desired. Open fencing (Figure 3), especially with bottom clearance, maximizes air flow and reduce reliance on air conditioning for cooling. However, if air conditioning rather than natural cooling is used to cool the home, prevailing summer winds should be blocked or diverted away from the house to reduce warm air infiltration. In general, vegetation provides greater flexibility in directing air circulation, and is a better choice than fencing used expressly for this purpose. In summer, large roof overhangs can help shade windows and walls, as well as walkways adjacent to the house. Arbors or trellises over outdoor living areas increase comfort and shade nearby walls. Decks should be built with bottom clearance to allow air to circulate below the structure. If possible, driveways should be located on the east or north side of the house to reduce heat buildup during warm afternoons. Solid surfaces such as concrete and asphalt, which transfer a great deal of heat, should be kept to a minimum, and ground cover plants or wood chip mulch used instead. Brick driveways build up less heat than either asphalt or concrete and produce less glare than concrete.

LANDSCAPE ELEMENTS FOR SHADE

Chapter IX. Landscaping to Conserve Energy: A Guide to Microclimate Modification

Overview Plants provide the most economical means of modifying microclimate around a home, and represent an investment in future energy savings. Research in Florida has shown that energy costs for air conditioning at certain times of the day can be reduced more than 50 percent by proper placement of the right plant materials around a residence. Plants in the landscape interact directly with the two primary comfort factors of Florida weather: sun and wind. Summertime heat gain in a home can be reduced by using plants to 1) shade the residence from direct solar radiation, 2) either divert or channel air movement away from or towards the house, and 3) create cooler temperatures near the home by evaporation of water from plant leaves. Heating costs in winter can be decreased by selecting and properly locating plants so that 1) the amount of direct solar radiation received by the home is maximized and 2) the effects of cold winter winds are minimized. In many areas of Florida, the wasteful practice of completely clearing building sites of existing vegetation is being abandoned. Existing trees should be incorporated into new home design whenever possible. Mature stands of native vegetation can often provide the desired energy saving shade and wind control that would otherwise require years to develop from new plantings of nursery stock. Native plants are well adapted to local climate and soils, and established natives will not ordinarily require supplemental irrigation or fertilization.

Using Trees for Shade Trees are the most valuable tools in landscaping for passively increasing the interior comfort of a home. Once established, most landscape trees require little periodic maintenance, and represent an appreciating investment in the home’s value. Trees are both effective providers of shade and modifiers of air movement. A tree species performance of these functions depends on how tall it grows, whether or not the leaves stay on the tree all year, and the shape and density of the canopy. Lists of appropriate trees and how they rate for these specific qualities may be found in three other publications: 1) EES-40 -- "Landscaping To Conserve Energy: Trees for North Florida," 2) EES-41 -"Landscaping to Conserve Energy: Trees for Central Florida," and 3) EES-42 -- "Landscaping to Conserve Energy: Trees for South Florida." These publications are available at your county extension office.

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Trees that grow 60 or more feet tall are capable of casting shadows over the roof of a typical single family house. Unfortunately, new plantings of most largegrowing trees require 20 or more years to reach full size. The homeowner is probably better off investing in quality ceiling insulation, attic ventilation, and construction of radiant barriers (a layer of aluminum foil situated in the air space between the roof and the attic insulation) unless a preexisting tree canopy effectively shades the roof during the summer months (Fiarey, 1984a; 1984b). Large trees overhanging the roof of a house also present the risk of damage from falling limbs, and the nuisance of clogging rain gutters with leaf and twig litter. If a solar collector has been installed on the roof, careful attention must be paid to positioning shade trees so that the efficiency of the collector is not reduced. Trees can provide valuable shading of sidewalls, particularly in older homes where walls have little insulation and retrofitting is prohibitively expensive. Small (up to 25 ft.) or medium size (25-40 ft.) trees perform this function well and don’t grow out of bounds. Fast growing trees (e.g., silk tree: Albizia julibrissin) can be planted at the same time as slower growing species to provide a temporary solution to shading problems. Such trees can be removed later as the slower growing trees approach their mature heights. Care must be taken that these vigorous, temporary trees do not shade or otherwise compete with the slower growing, more permanent landscape elements. Eastern and western exposures accumulate the most heat during the long days of summer. Tree shading should thus be maximized on these sides of the house. South-facing walls also benefit from tree shade. Although southern exposures may be relatively free of direct radiation in June, by August the sun has dropped sufficiently in the sky to cause significant heat load increase in the afternoon. Windows are the most direct route for sunlight to enter the home, and trees (or shading devices such as awnings) should be positioned to shade them throughout the day. The outdoor compressor/condenser unit of the air conditioning system uses less energy if it and the surrounding area are shaded from direct sun during the entire day. A tree can shade the unit when the sun is overhead, while nearby shrubs can provide protection during the early morning and late afternoon hours. However, care must be taken not to block the conditioner’s air flow ("short circuiting"). If the warm discharge air is prevented from escaping, the intake air

Chapter IX. Landscaping to Conserve Energy: A Guide to Microclimate Modification

temperature will be raised, causing the unit to operate less efficiently. In winter, the sun is low in the southern sky. Southern exposures of a home can receive the cost-free, energy saving benefits of passive solar heating, provided that deciduous trees are used along the southern exposures (Figure 4). Deciduous trees shed their leaves in the fall, and are bare during the coldest months of the year. However, in south Florida, where winters are short and mild, greater benefits are received by using evergreen trees along the southern exposures to reduce heat build-up and lower air conditioning costs.

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Trees planted close to the home begin to provide shade sooner than those planted at greater distances. The benefits of new shade trees should be obtained within five years. To accomplish this goal, a distance of seven to twenty feet from tree to sidewall is recommended. Lot size and ultimate height of the tree chosen directly influence this distance. Trees planted closer also shade for a longer period of time during the day, and over a greater part of the hot season. The shadow of a tree planted ten feet from the home moves across the target surface four times slower than a tree planted twenty feet away (Parker, 1978; 1983a; 1983b). Vegetation close to the residence also lowers the air temperature near the home, reducing the heat conducted through the walls.

Figure 5. The shape of a tree canopy strongly influences the shade pattern that is cast.

Figure 4. Deciduous trees on the southern exposure of the home provide shade in summer, but allow winter sunlight to passively heat the home.

Evergreen trees maintain their leaves throughout the year. There are two types: broad-leaved (e.g., Southern magnolia, Magnolia grandiflora; American holly, Ilex opaca), and needle-leaved (e.g., pines, Pinus spp.; cedars and junipers, Juniperus spp.). Evergreens that are broadleaved provide dense shade year round, and are most useful as shade trees in the southern third of Florida. By contrast, the shade cast by needle-leaved trees is sparse and more open, though pruning can, in some cases, stimulate a denser canopy to develop. Where allowance for air circulation must be balanced with degree of shading, more open canopied trees may be preferred when air circulation and the degree of shading must be optimized for maximum energy savings and comfort. Tree shape also influences the amount of shade cast (Figure 5).

The correct placement of trees chosen to shade the home involves consideration of the angle of the sun’s rays, the mature height and width of the tree canopy, and the height of the structure to be shaded. Precise guidelines on determining shade patterns can be found in the following three publications: 1) EES-49 -"Landscaping to Conserve Energy: Shade Patterns in North Florida," 2) EES-50 -- "Landscaping to Conserve Energy: Shade Patterns in Central Florida," and 3) EES48 -- "Landscaping to Conserve Energy: Shade Patterns in South Florida." These publications are available at your county extension office.

Using Shrubs and Vines for Shade Shrubs can provide effective barriers to early morning and late afternoon sunlight on eastern and western exposures respectively. Small-leaved, openbranched shrubs provide shade without unduly restricting air movement for passive cooling. Espaliered shrubs, shrubs trained to grow horizontally against a wall block

Chapter IX. Landscaping to Conserve Energy: A Guide to Microclimate Modification

a great deal of sunlight before it strikes and heats up the wall (Figure 6).

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Winter In Florida, winds prevail from the north in winter. These northerly winds often sweep cold, arctic air into the state, the effects of which are felt most strongly in the Panhandle and north-central counties.

Figure 6. Espaliered shrubs can insulate a wall from heat build-up.

Vines are especially useful for shading homes when small lot size restricts the use of shade trees. Vines are either self-supporting or twining. Self-supporting vines cling to a surface by either pad-like holdfasts (e.g., Virginia creeper: Parthenocissus quinquefolia) or aerial roots (e.g., trumpet vine: Campsis radicans). Selfsupporting vines are not recommended for wood structures because they may trap moisture that can lead to decay of the wood. On brick or concrete block homes, a fast growing, self-supporting vine can effectively prevent the sun from heating a wall (Figure 6) (Parker, 1981). Twining vines (Confederate jasmine: Trachelospermum jasminoides; painted trumpet: Clystostoma callistegioides) climb by means of stems or tendrils that require some form of support. By providing lattice-type support or a trellis, twining vines can be used to shade walls, windows, and outdoor living spaces (Figure 7). As with shade trees, only deciduous vines are recommended for southern exposures on structures in north Florida, to allow winter sun to passively heat the home.

LANDSCAPE ELEMENTS FOR WIND CONTROL

Figure 7. Twining vines provided with climbing support shade an outdoor living space, and block the sun from nearby windows and walls.

A home loses a greater amount of heat on a cold and windy day than on an equally cold but still day. About 1/3 of the heat loss from a home is by transfer through the ceilings and walls (conduction). Wind increases the loss of heat from the outside surfaces of those same walls and from the roof by sweeping the warm air away (convection). Cold air infiltration through spaces around windows and doors also increases reliance on costly home heating systems. The use of windbreaks and foundation plantings substantially reduces the heat robbing action of winter winds. In Florida, windbreaks situated on the north, northwest and, to a lesser extent, northeast exposures of the home can provide significant energy savings during the winter heating season. The height and foliage density of trees used in windbreaks directly influence their effectiveness as wind barriers. Evergreen trees with dense canopies provide the most complete protection. However, extremely dense or solid windbreaks tend to concentrate their effects over a much shorter distance

Chapter IX. Landscaping to Conserve Energy: A Guide to Microclimate Modification

than those of moderate texture. A multi-layered canopy of shrubs and trees of moderate density planted in two to five rows (Figure 8) is the most effective windbreak design, but even a single row of trees provides some windbreak action. Windbreaks significantly reduce wind velocity for a distance equal to 10 times the height of the trees, less significantly to 20 times the height. The greatest amount of protection occurs within a distance of five times the height of the windbreak. A dense planting of shrubs close to the north and northeastern walls of the home creates a "dead air" space that has insulating properties (Figure 8). By reducing air movement in the immediate vicinity of the walls, this dead air space also reduces secondary heat loss by cold air infiltration through cracks and window spaces. Evergreen shrubs of dense texture (e.g., Podocarpus nagi, Pittosporum tobira), closely spaced together, provide this type of protection effectively. These same shrubs keep north sidewalls cool in the summer via transpiration and shading during early morning and late afternoon.

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largely determined by the means with which the home is cooled. For a home in which air conditioning is used only minimally, trees and shrubs should be strategically situated to channel cooling breezes toward the windows (Figure 9). Low-branching trees should be avoided on the southeastern and/or southwestern exposure, or the low branches removed (Figure 10). Plants used to shade windows from the sun should be far enough away to not restrict air movement. Shrubs near the windows can be positioned to further funnel moving air into the house (Figure 9). If shrubs are be used to provide low shade for exposures facing prevailing summer winds, use species that have small leaves and an open branching pattern (e.g., Glossy abelia: Abelia X grandiflora in northern Florida; Thryallis: Galphimia glauca in south or central Florida). Winter wind barriers on the north and northwest sides of the home also deflect cooling breezes from the south back toward the house in the summer (Figure 8). During the 5 to 7 months of Florida’s uncomfortably warm temperatures, some residents find it impossible to stay cool without air conditioning, regardless of the cost. Wind movement around the home during the cooling season substantially raises the energy costs for air conditioning by increasing the infiltration of hot, humid outside air around windows, doors, and through cracks. Studies of air conditioned homes in Florida have determined that heat gain by infiltration is actually greater than gain by conduction and radiation through walls and windows (Steen et al., 1976). Shrubs and trees should be positioned around the air conditioned home to divert the prevailing southern breezes away from the house (Figure 12), the exact

Figure 8. Windbreaks and foundation plantings will weaken the effects of cold winter winds.

Summer In Florida, summer breezes prevail from the south and southeast. In north Florida, breezes during the "dog days" of July and August originate from the south or southwest; in south Florida they largely remain southeasterly (Barrick and Black, 1984). How best to use plants to interact with summer air movement is

Figure 10. To allow air movement around passively cooled homes, low-branching trees should be avoided on the southeastern and/or southwestern exposure, or the low branches removed.

Chapter IX. Landscaping to Conserve Energy: A Guide to Microclimate Modification

Figure 9. Around the passively cooled home, trees and shrubs should be positioned to direct breezes toward windows.

opposite of what would be desired for a passively cooled home. A multi-layered summer windbreak should be designed along the southern exposures and away from the home. The tallest components of the windbreak should be the closest to the house. In this way, a "wind ramp" can be created that will channel the breezes over the home. Along and close to the walls that face the direction of summer winds, a foundation planting of shrubs should be used to create a dead air space that will reduce or eliminate warm air infiltration. Deciduous shrubs, or at least more open-branched species than recommended for north-facing foundation plantings, should be used on the south side to allow the sun to passively heat those exposures in winter. During the mild transitional months of fall and early spring, natural ventilation is desirable, even in homes that are air conditioned during the peak of the hot season. The south-facing foundation shrubs can be pruned in September to permit air movement, and then allowed to fill out again the following spring, if such pruning does not disrupt the aesthetic integrity of the landscape. Shade trees positioned between windows and prevailing summer winds should be low-branching (Figure 11) to provide maximum protection against air movement. Additional tall shrubs can be placed nearby on the windward side of east and west windows.

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Figure 11. If the home is air-conditioned, shade trees positioned between windows and prevailing summer winds should be low-branching to provide maximum protection against air movement.

COOLING EFFECTS OF TRANSPIRATION Plants release water through pores in their leaves. The evaporative loss of this water is called transpiration. As hot air passes over the surface of the leaves, the moisture absorbs some of the heat and evaporates. The air surrounding the leaf surface is thus cooled by the process. This interaction is called evaporative cooling, and air temperatures surrounding vegetation can be lowered by as much as 9 degrees F (5 degrees C) by its effects. The greater the amount of leaf area in the landscape, the greater the cooling effects of transpiration. The use of plants for shade and wind control instead of structural features such as fences and arbors thus provides an additional benefit toward maintaining thermal comfort during Florida’s long summer. Air temperatures near shade trees and foundation shrubs will be considerably lower than open areas, resulting in lower heat gains through nearby walls or windows. If summer breezes are channeled through and across vegetation, their cooling capacity will be increased. To maximize the effects of evaporative cooling, increase the amount of plant cover around the home. Use turf and/or ground covers to their fullest potential in the landscape, as alternatives to paved surfaces such as asphalt or concrete. Many ground covers require less maintenance than turf grasses, and can be used as energy saving alternatives to large expanses of lawn. Suggested

Chapter IX. Landscaping to Conserve Energy: A Guide to Microclimate Modification

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Shade the outside compressor unit from direct sun. Select trees to block air flow to the house. Situate a windbreak at some distance from the house to divert or block prevailing winds away from the home. Reduce warm air infiltration with a solid foundation planting that creates a dead air space near the house in the face of prevailing winds. To minimize energy needs for winter heating: Figure 12. Multi-layered windbreak of shrubs and trees positioned around air-conditioned home divert southern breezes away from house.

plant materials and guidelines for using ground covers may be found in the following three publications: 1) EES-37 -- "Landscaping to Conserve Energy: Ground Covers for North Florida," 2) EES-38 -- "Landscaping to Conserve Energy: Ground Covers for Central Florida," and 3) EES-39 -- "Landscaping to Conserve Energy: Ground Covers for South Florida." These publications are available from your county extension office.

SUMMARY

1. Use deciduous trees, shrubs and vines on south, southeast and southwest locations except in south Florida. 2. Maximize the use of evergreen plant materials in foundation plantings to create an insulating dead air space along the northern exposure of the house. 3. Create a windbreak for the north and northwestern exposures of the home using evergreen trees and shrubs in a multi-layered canopy, preferably in 2- to 5-foot rows. With careful attention to these guidelines, the Florida homeowner can improve the value of his or her home with landscaping and realize substantial energy savings throughout the year.

BIBLIOGRAPHY AND REFERENCES FOR FURTHER READING

In order to reduce energy needs for summer cooling: 1. Maximize use of ground covers and turf; limit amount of dark paved areas. 2. Maximize shading on roof by overhead canopy trees (provided that solar panels are not shaded). 3. Maximize shading on east, west, and south walls with shade trees. Shadows on the south, southeast, and southwest exposures are shorter than shadows cast on west and east exposures; position trees and shrubs accordingly. 4. Use trellises in combination with ornamental vines for eastern, western, and southern exposures. 5. If air conditioning is used only minimally, maximize the beneficial effects of prevailing cooling winds. Select and prune trees to allow air flow to the house. Avoid dense plantings away from the house that would act as a windbreak to cooling breezes. Avoid the use of solid foundation plantings that would create a dead air space. 6. If you rely on air conditioning:

Barrick, W.E. and R.J. Black. Florida Climate Data. 1984. Florida Energy Extension Service Publication EES-5, University of Florida, Gainesville. Buffington, D.E. 1979. Economics of Landscaping Features for Conserving Energy in Residences. Proceedings of the Florida State Horticultural Society 92:216-220. Fairey, P.F. 1984a. Radiant Energy Transfer and Radiant Barrier Systems in Buildings. Design Note 6, Florida Solar Energy Center, Cape Canaveral. Fairey, P.F. 1984b. Designing and Installing Radiant Barrier Systems. Design Note 7, Florida Solar Energy Center, Cape Canaveral. Parker, J.H. 1978. Precision Landscaping for Energy Conservation. Proceedings of National Conference on Technology for Energy Conservation. Tucson, Arizona.

Chapter IX. Landscaping to Conserve Energy: A Guide to Microclimate Modification

Parker, J.H. 1981. A Comparative Analysis of the Role of Various Landscape Elements in Passive Cooling in Warm, Humid Environments. Pp 365-368 in Proceedings of the International Passive and Hybrid Cooling Conference, Miami.

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Parker, J.H. 1983a. The Effectiveness of Vegetation on Residential Cooling. Passive Solar Journal. 2:123132. Parker, J.H. 1983b. Landscaping to Reduce the Energy Used in Cooling Buildings. J. Forestry 81:82-84, 105. Steen, J.W. Shrode and E. Stuart. 1976. Basis for Development of a Viable Energy Conservation Policy for Florida Residents. Florida State Energy Office, Tallahassee.